1. Field of the Invention
The present invention pertains to the stimulation of crude oil reservoirs to enhance production using a combination of pulsed power electrohydraulic and electromagnetic methods and the processing of the recovered crude oil into its components. In particular, the present invention provides a method and apparatus for recovery of crude oil from oil bearing soils and rock formations using pulsed power electrohydraulic and electromagnetic discharges in one or more wells that produce acoustic and coupled electromagnetic-acoustic vibrations that can cause oil flow to be enhanced and increase the estimated ultimate recovery from reservoirs.
2. Background of the Invention
The stimulation of crude oil reservoirs to enhance oil production from known fields is a major area of interest for the petroleum industry. One of the single most important research goals in fossil fuels is to recover more of the hydrocarbons already found. At present, approximately 66% of discovered oil is left in the ground due to the lack of effective extraction technology for secondary and tertiary Enhanced Oil Recovery (EOR). A EOR technology that can be deployed easily and at low cost in onshore and offshore field locations would greatly improve the performance of many oil fields and would increase significantly the world""s known recoverable oil reserves.
Methods that are widely used for the purpose rely on the injection of fluid at one well, called the injection well, and use of the injected fluid to flush the in situ hydrocarbons out of the formation to a producing well. In one mode of secondary recovery, a gas such as CO2 that may be readily available and inexpensive, is used. In other modes, water or, in the case of heavy oil, steam may be used to increase the recovery of hydrocarbons. One common feature of such injection methods is that once the injected fluid attains a continuous phase between the injection well and the production well, efficiency of the recovery drops substantially and the injected fluid is unable to flush out any remaining hydrocarbons trapped within the pore spaces of the reservoir. Addition of surfactants has been used with soome success, but at high cost, both economic and environmental.
Many methods have been developed that try address the problem of driving out the residual oil. They can be divided into a number of broad categories.
The first category uses electrical methods. For example, U.S. Pat. No. 2,799,641 issued to Bell discloses a method for enhancing oil flow through electrolytic means. The method uses direct current to stimulate an area around a well, and uses the well-documented effect known as electro-osmosis to enhance oil recovery. Another example of electro-osmosis is described in U.S. Pat. No. 4,466,484 issued to Kermabon wherein direct current only is used to stimulate a reservoir. U.S. Pat. No. 3,507,330 issued to Gill discloses a method for stimulating the near-wellbore volume using electricity passed upwards and downwards in the well using separate sets of electrodes. U.S. Pat. No. 3,874,450 issued to Kern teaches a method for dispersing an electric current in a subsurface formation by means of an electrolyte using a specific arrangement of electrodes. Whitting (U.S. Pat. No. 4,084,638) uses high-voltage pulsed currents in two wells, a producer and an injector, to stimulate an oil-bearing formation. It also describes equipment for achieving these electrical pulses.
A second category relies on the use of heating of the formation. U.S. Pat. No. 3,141,099 issued to Brandon teaches a device installed at the bottom of a well that causes resistive heating in the formation though dielectric or arc heating methods. This method is only effective within very close proximity to the well. Another example of the use of heating a petroleum bearing formation is disclosed in U.S. Pat. No. 3,920,072 to Kern.
A third category of methods relies on mechanical fracturing of the formation. An example is disclosed in U.S. Pat. No. 3,169,577 to Sarapuu wherein subsurface electrodes are used to cause electric impulses that induce flow between wells. The method is designed to create fissures or fractures in the near-wellbore volume that effectively increase the drainage area of the well, and also heat the hydrocarbons near the well so that oil viscosity is reduced and recovery is enhanced.
It has long been documented that acoustic waves can act on oil-bearing reservoirs to enhance oil production and total oil recovery. A fourth category of methods used for EOR rely on vibratory or sonic waves, possibly in conjunction with other methods. U.S. Pat. No. 3,378,075 to Bodine discloses a method for inducing sonic pumping in a well using a high-frequency sonic vibrator. Although the sonic energy generated by this method is absorbed rapidly in the near wellbore volume, it does have the effect of cleaning or sonicating the pores and fractures in the near-wellbore area and can reduce hydraulic friction in the oil flowing to the well. Another example of a vibratory only technique is disclosed by U.S. Pat. No. 4,049,053 to Fisher et al. wherein several low-frequency vibrators are installed in the well and are driven hydraulically using surface equipment. U.S. Pat. No. 4,437,518 issued to Williams describes the design for a piezoelectric vibrator that can be used to stimulate a petroleum reservoir. U.S. Pat. No. 4,471,838 issued to Bodine teaches a method for using surface vibrations to stimulate oil production. The surface source defined in this patent is not sufficient to produce significant enhanced recovery of crude oil.
Turning next to methods that use vibratory or sonic waves in conjunction with other methods, U.S. Pat. No. 3,754,598 to Holloway, Jr. discloses a method that utilizes at least one injector well and another production well. The method imposes oscillating pressure waves from the injector well on a fluid that is injected to enhance oil production from the producing well. U.S. Pat. No. 2,670,801 issued to Sherborne discloses the use of sonic or supersonic vibrations in conjunction with fluid injection methods: the efficiency of the injected fluids in extracting additional oil from the formation is improved by the use of the acoustic waves. U.S. Pat. No. 3,952,800, also to Bodine teaches a sonic treatment in which a gas is injected into the well and is used to treat the wellbore surface using sonic wave stimulation. The method causes the formation to be heated through the gas by heating from the ultrasonic vibrations. U.S. Pat. No. 4,884,634 issued to Ellingsen uses vibrations of an appropriate frequency at or near the natural frequency of the formation to cause the adhesive forces between the formation and the oil to break down. The method calls for a metallic liquid (mercury) to be placed in the wells to the level of the reservoir and the liquid is vibrated while also using electrodes placed in the wells to electrically stimulate the formation. Apart from the potential environmental hazards associated with the handling and containment of mercury, this method faces the problem of avoiding formation damage due to an excess of borehole pressure over the formation fluid pressure caused by the presence of a dense liquid. U.S. Pat. No. 5,282,508, also issued to Ellingsen et al. defines an acoustic and electrical method for reservoir stimulation that excites resonant modes in the formation using AC and/or DC currents along with sonic treatment. The method uses low frequency electrical stimulation.
The success of the existing art in stimulating reservoirs has been spotty at best, and the effective range of such methods has been limited to less than 1000 feet from the stimulation source. A good discussion on wettability, permeability, capillary forces and adhesive and cohesive forces in reservoirs is provided by the Ellingsen ""508 patent. These discussions fairly represent the state of knowledge on these subjects and are not repeated herein. These discussions do not, however, address the limitations on the current state of the art in acoustic stimulation.
Existing acoustic stimulation methods have demonstrated clearly that they are limited to a range of about 1000 feet from the stimulation point. This limit is caused by the natural attenuation properties of the reservoir, which absorb high frequencies preferentially and reduce the effective frequency range to less than a few hundred Hertz at distances beyond about 1000 feet from the acoustic source. This same limit has plagued seismic imaging in cross-borehole studies for many years and is a fundamental physical limitation on all acoustic methods.
Effective acoustic stimulation of oil-bearing reservoirs requires support at greater distances from the stimulation source than possible with most of the prior art. In addition, there is some empirical evidence suggesting that higher frequencies than direct acoustic methods can generate may be more effective in stimulation of oil-bearing reservoirs. Accordingly, it is desirable to have a stimulation source that has a greater range of effectiveness than the prior art discussed above. Such a source should preferably be able to provide stimulation at higher frequencies than the 10-500 Hz typically attainable using prior art methods.
U.S. Pat. No. 4,345,650 issued to Wesley teaches a device for electrohydraulic recovery of crude oil using by means of an electrohydraulic spark discharge generated in the producing formation in a well. This method presents an elegant apparatus that can be placed in the producing interval and can produce a shock and acoustic wave with very desirable qualities. The present invention will build on the teachings of this patent and will extend the effective range of Wesley""s method through new and novel equipment designs and field configurations of Wesley""s apparatus and new apparatus designed to enhance the effect on oil reservoirs.
Hydrocarbons recovered from a wellbore may include a number of components. The term xe2x80x9ccrude oilxe2x80x9d is used to refer to hydrocarbons in liquid form. The API gravity of crude oil can range from 6xc2x0 to 50xc2x0 API with a viscosity range of 5 to 90,000 cp under average conditions. Condensate is a hydroacarbon that may exist in the producing formation either as a liquid or as a condensable vapor. Liquefaction of the gaseous components occurs when the temperature of the recovered hydrocarbons is lowered to typical surface conditions. Recovered hydrocarbons also include free gas that occurs in the gaseous phase under reservoir conditions, solution gas that comes out of solution from the liquid phase when the temperature is lowered, or as condensable vapor. Recovered hydrocarbons also commonly include water that may be in either liquid form or vapor (steam). The liquid water may be free or emulsified: free water reaches the surface separated from liquid hydrocarbons whereas the emulsified water may be either water dispersed as an emulsion in liquid hydrocarbons or as liquid hydrocarbons dispersed as an emulsion in water. Produced well fluids may also include gaseous impurities including nitrogen, helium and other inert gases, CO2, SO2 and H2S. Solids present in the recovered wellbore fluids may include sulphur. Heavy metals such as chromium, vanadium or manganese may also be present in the recovered fluids from a wellbore, either as solids or in solution as salts. In all enhanced EOR operations, it is desirable to separate these and other commercially important materials from the recovered fluids.
The present invention is a pulsed power device and a method of using the pulsed power device for EOR. Pulsed power is the rapid release of electrical energy that has been stored in capacitor banks. By varying the inductance of the discharge system, energies from 1 to 100,000 Kilojoules can be released over a pulse period from 1 to 100 microseconds. The rapid discharge results in a very high power output that can be harnessed in a variety of industrial, chemical, or medical applications. The energy release from the system can be used either in a direct plasma mode through a spark gap or exploding filament, or by discharging the energy through a single- or multiple-turn coil that generates a short-lived but extremely intense magnetic field.
When electricity stored in capacitors is released across a spark gap submerged in water, a plasma channel is created that vaporizes the surrounding water. This plasma ionizes the water and generates very high pressures and temperatures as it expands outward from the discharge point. In a plasma, or electrohydraulic (EH) mode, the pulse may be used in a wide range of processes including geophysical exploration, mining and quarrying, precision demolition, machining and metal forming, treatment and purification of a wide range of fluids, ice breaking, defensive weaponry, and enhanced oil recovery which is the purpose of the present invention. The basic physics of the shock wave that is generated by the EH discharge is well understood and is documented in U.S. Pat. No. 4,345,650 issued to Wesley, and incorporated herein by reference.
In the electromagnetic (EM) mode, the coil is designed to produce controlled flux compression that can be used to generate various physical effects without the coupled effect of the EH strong acoustic wave. In both systems, however, typical systems require about 0.5 to 1 seconds to accumulate energy from standard power sources. The ratio of accumulation time to discharge time (100,000 to 1,000,000) allows the generation of pulses with several gigawatts of peak power using standard power sources.
Given the physical limitations on direct acoustic stimulation caused by attenuation in natural materials, acoustic stimulation must be generated using wide band vibrations in these materials at distances much greater than the current limitation of about 1000 feet. The present invention addresses this issue in a new and innovative way using pulsed power as the source. The Wesley ""650 patent teaches a method for generating strong acoustic vibrations for reservoir stimulation that has been shown in the field to have an effective limit of about 1000 feet. What was not recognized in the Wesley teachings was that the pulsed power method also has a unique ability to generate high-frequency acoustic stimulation of the reservoir separately from the direct acoustic response of the EH shock wave generated by the plasma discharge in the wellbore. In addition to the direct shock wave effect claimed in the Wesley patent, the pulsed power discharge also generates a strong electromagnetic pulse that travels at the speed of light across the reservoir. As this electromagnetic pulse transits the reservoir, it induces a coupled acoustic vibration at very high frequencies in geologic materials like quartz that causes stimulation at multiple scales in the reservoir body. This induced acoustic vibration acts for a short period of time after the pulse is discharged, usually on the order of about 0.1 to 0.3 seconds, but is induced everywhere that the electromagnetic pulse travels. Thus, it is not limited by the natural acoustic attenuation that limits the effectiveness of a direct acoustic pulse source because it is induced at all locations in-situ by the electromagnetic pulse. At the same time, the lower-frequency direct acoustic pulse travels through the reservoir at the velocity of sound. This direct acoustic pulse assists the electromagnetically-induced vibrations in stimulating the reservoir, but has a clearly limited range due to the finite speed that it can travel before the EM-induced vibrations decay and become ineffective.
Effective acoustic stimulation of oil-bearing reservoirs requires higher frequencies than direct acoustic methods can generate and support at great distances from the stimulation source. Every rock formation can be modeled as a uniform equivalent medium with imbedded inclusions. These inclusions can be present at the pore scale, grain scale, crack scale, lamina scale, bedding scale, sand body scale, and larger scales. Each of these inclusions, or features, of the formation act as scatterers that absorb acoustic energy. The frequency of the energy absorbed is directly correlated to the scale of the inclusions and the contrast in physical properties between the inclusion and the surrounding matrix, and this absorption provides the energy for enhanced oil recovery that is required at a specific scale of inclusion. Hence, an effective acoustic stimulation program can be designed to optimize the energy absorption and effective stimulation if the scale of the inclusions and their physical properties are known, and if the acoustic stimulation frequencies can be targeted at these inclusion scales over a large volume of the reservoir. The limitations and variations in the effectiveness of existing acoustic methods are directly correlated to the narrow band of seismic frequencies from 10-500 hertz used to stimulate and whether there are inclusions at those frequencies within the effective range of the stimulation method in question. When this physical understanding of the role of acoustic absorption by scale dependent features in reservoirs is included, it becomes readily apparent why existing acoustic methods with a frequency band limited to a few hundred hertz are not capable of stimulating most reservoirs effectively. The existing technology has demonstrated a spotty record because the narrow band of frequencies used are often not the right ones for stimulating the critical inclusions of a particular reservoir. The scale of the inclusions that are critical to effective stimulation exist at the pore scale, grain scale, flat-crack scale, and fracture scale, all of which are activated by much higher frequencies (kilohertz and higher) than the band pass of the low-frequency direct acoustic wave.
The present invention differs from all of the prior art in several ways. First, it uses a coupled process of direct EH acoustic vibrations that propagate outward into the formation from one or more wells, and electromagnetically-induced high-frequency acoustic vibrations that are generated using both EH and EM pulsed power discharge devices that takes advantage of the acoustic coupling between the electromagnetic pulse and the formation. This is significantly different from the prior art which relies on acoustic vibrations only, or a combination of acoustic vibrations and low-frequency AC or DC electrical stimulation.
The present invention also recognizes that these two effects must occur together to effectively mobilize the oil and increase production of the oil. The problem that arises is that the EM-induced vibrations only occur for a short time after the electrohydraulic or electromagnetic pulse is initiated. The electrohydraulic acoustic pulse travels at a finite speed from the well where the pulse originates, so that the effective range of the technique is defined by how far the acoustic wave can travel before the electromagnetically-induced vibration in the reservoir ceases. Hence, a single pulse source has a range that is limited by the pulse characteristics employed.
In a preferred embodiment of the present invention, the technique can be applied using a multi-level discharge device that allows sequential firing of several sources in one well in a time sequence that is optimized to allow continuous electromagnetic-coupled stimulation of a large reservoir volume while the electrohydraulic acoustic pulse travels further from the pulse well than it could before a single source electromagnetic vibration would decay. This approach can be used to extend the effective range of the stimulation by a factor of 5-6 from about 1000 feet as claimed and proven in the Wesley patent, i.e., up to distances of 5000 to 6000 feet claimed in the present invention. This allows the technique to be applied effectively to a wide range of oil fields around the world. This concept can be extended to the placement of multiple tools in multiple wells to achieve better stimulation of a specific volume of the reservoir.
In another embodiment of the invention, the range of the technique is extended by using multiple pulse sources in multiple wells that allow the electromagnetically-induced vibrations to continue for a longer time, thus allowing the acoustic pulse to travel further into the formation, effectively extending the range of coupled stimulation that can be achieved. This embodiment utilizes a time-sequential discharge pattern that produces a series of electromagnetically-induced vibrations that will last up to several seconds while the direct acoustic pulse travels further from the discharge source to interact with the electromagnetically-induced vibrations at much greater distances in the reservoir.
In another embodiment of the present invention, multiple EH and EM sources can be placed in multiple wellbores and discharged to act as an array that will stimulate production of the oil in a given direction or specific volume of the reservoir.
In another aspect of the invention, the discharge characteristics of the pulse sources can be customized to produce specific frequencies that will achieve optimal stimulation by activating specific scales of inclusions in the reservoir. In this embodiment, the discharge devices can have their inductances modified to achieve a variety of pulse durations and peak frequencies that are tuned to the specific reservoir properties. This allows for the design of a multi-spectral stimulation program that can activate those inclusions that are critical to enhanced production, while preventing activation of those inclusions that might inhibit enhanced production. Once the desired inclusions for stimulation are defined by conventional geophysical logging methods, a reservoir model is constructed and the optimal frequencies for the stimulation are determined. The pulse tool can be adapted to a wide range of pulse durations and peak frequencies by adjusting the induction of the capacitor circuits in the pulse tool. Where multiple frequencies are desired to achieve stimulation at several scales, the multi-level tool in a single well or multiple tools placed in multiple wells can be tuned to the reservoir to optimize the desired stimulation effect and produce a multi-spectral stimulation of the reservoir.
The present invention also differs from the previous art in that it includes the use of EM pulse sources that do not generate a direct acoustic shock pulse like the plasma shock effect caused by the spark gap in the electrohydraulic device defined by Wesley. These pulse sources replace the conventional spark gap discharge device defined by Wesley with a single-turn magnetic coil that produces a magnetic pulse with no acoustic pulse effect. This tool can be placed in more sensitive wells that will not tolerate the strong shock effect of an EH pulse generator. They also allow a wider range of discharge pulse durations that will extend the effective frequency range of induced vibrations that can be applied to a given reservoir.
In another embodiment of the present invention, the EH pulse source can be directed using a range of directional focusing and shaping devices that will cause the acoustic pulse to travel only in specific directions. This reflector cone allows the operator to aim the pulses from one or multiple wells so that they can effect the specific portion of the formation where stimulation is desired.
In another embodiment of the present invention, the pulse source is placed in an injector well that is being used for water injection, surfactant injection, diluent injection, or CO2 injection. The tool can be configured to operate in a rubber sleeve to isolate it, where appropriate, from the fluids being injected. The tool can be deployed in a packer assembly suspended by production tubing, and can be bathed continuously in water to maintain good coupling to the formation. Gases generated by the electrohydraulic discharge can be removed from the packer assembly by pumping water down the well and allowing the gases to be flushed back up the production tubing to maintain optimal coupling and avoid the increase in compressibility that would occur if the gases were left in the well near the discharge device.
A chronic problem with electrohydraulic discharge devices is that the electrodes are prone to wear and must be replaced from time to time. In another embodiment of the present invention, the electrodes designed for electrohydraulic stimulation have been improved using several methods including (1) improved alloys that withstand the pulse discharge better and last longer, (2) two new feeding devices for exploding filaments, one with a hollow electrode using a pencil filament, and one with a rolled filament on a spool, that allows the exploding filament to be threaded across the spark gap rapidly between discharges so that the pulse generator can operate more efficiently, and (3) gas injection through a hollow electrode that acts as a spark initiation channel.
In another embodiment of the invention, the fluids produced from the wellbore are separated into its components. These components may include one or more of associated gas, condensate, liquid hydrocarbons, helium and other noble gases, carbon dioxide, sulphur dioxide, pyrite, paraffins, heavy metals such as chromium, manganese and vanadium.